All-optical signal processing method and device

ABSTRACT

A method and an optical device in all-optical signal processing. It provides a novel way of taking into use the potential bandpass filtering capabilities of an optical complex one-pole resonator by, 1) excitating with at least part of the input signal an optical resonator arrangement that comprises two substantially parallel, independent, complex one-pole resonators arranged in a manner that one of the resonators is matched, and the other one is non-matched with the input signal, and 2) based on polarization separating further from the output of the optical resonator arrangement at least one optical output signal so that both the matched and non-matched resonators contribute to the formation of the output signal. The method and device can be utilized, for example, for optical signal analysis, optical clock recovery, or for producing high-frequency outputs from lower frequency input signals, like e.g. optical microwave generation.

FIELD OF INVENTION

The present invention relates to a method in optical signal processingand to an optical device for carrying out said method. Moreparticularly, the invention relates to all-optical filters based on anoptical resonator arrangement, which has a capacity to store temporallythe electromagnetic energy of the incoming light and therefore has somememory about its past so that the filter can stay on a desired state forsome time regardless of fast perturbations on the optical input signal.Such a filter may be referred to as a “slow” all-optical filter. Theinvention can be utilized, for example, for optical signal analysis,optical clock recovery, or for producing high-frequency outputs fromlower frequency input signals, like e.g. optical microwave generation.

DEFINITIONS

The frequency of light is typically understood as the inverse of thewavelength of the electromagnetic field, i.e. ν=c/(nλ), where ν is thefrequency, c is the speed of light in vacuum, n is the index ofrefraction of the medium and λ is the wavelength. In the context of thisapplication, the word frequency, however, addresses to the rate oftemporal, periodic intensity change of the light (e.g. a pulsefrequency), and is therefore not connected to the wavelength. Tohighlight this conceptual difference, this rate of intensity change iscalled hereafter simply the frequency f, and the frequency ν as thefundamental physical property of the electromagnetic field is discussedin terms of wavelength λ. In other words, frequency f relates hereafterto temporal pulses of light, which pulses are formed from light havingwavelength λ, and frequency ν corresponding to said wavelength.

Due to historical reasons optical resonators are called sometimes asoptical cavities. These two expressions are used interchangeablythroughout the document.

BACKGROUND OF THE INVENTION

Electrical Signal Processing

Bandpass filtering is traditionally made with electrical means andoptical signals processed by conventional methods require thereforeoptical-to-electrical conversion before the filtering can be performed.If the signal is transmitted further by optical means, one has toconvert the filtered signal back to optical domain. Conversion to andfrom electrical domain is non-trivial with current state-of-the-artwhenever signal frequencies go above 40 GHz. The conversions forth andback generate also additional cost and possibly degrade optical signalquality. For example patent U.S. Pat. No. 4,737,970 discloses a clockrecovery device utilizing a cavity resonator in electrical domain. Thepresent invention aims to all-optical conversion requiring thus noconversions to and from electrical domain and eliminating therefore anyproblems related to said conversions.

All-Optical Signal Processing

All-optical signal processing traditionally deals with differentwavelengths of light, where e.g. different channels in WDM (WavelengthDivision Multiplexing) system are separated from each other usingoptical filters having different wavelength bands.

Optical processing based on temporal input signal frequencies, notwavelengths of light, has been done far less, but is also known fromprior-art. Ring-resonators have been used for construction of all-passoptical filters (Azana and Chen, IEEE Photonic Technology Letters, 14,2002) and together with Mach-Zehnder interferometers they typically areconsidered as basic building blocks of all-optical filters. Cascadedring-resonators and Mach-Zehnder interferometers can be used to performa Fourier-transform and other optical functions. Optical resonators havebeen used for all-optical signal processing (Lenz et al, IEEE J. QuantumElectron., 34, August 1998) and clock recovery (Jinno and Matsumoto,IEEE J. Quantum Electron., 4, April 1992). The clock recovery made byJinno was based on simple optical resonator, whose optical length wascarefully matched with: 1) wavelength of the incoming light and 2) datafrequency. The device is, however, very limited in its capabilities toprocess more than one wavelength (or WDM channel) at time.

Some other all-optical filtering techniques are presented in thefollowing patent publications. U.S. Pat. No. 5,446,573 discloses anall-optical regenerator that is based on the use of a non-linear ringresonator. U.S. Pat. No. 6,028,687 discloses a laser arrangement betweentwo resonator mirrors for recovering a clock from a modulated opticalinput signal. Here, bi-directional mux/demux components have beenarranged within the resonator. U.S. Pat. No. 6,388,753 discloses anall-optical bit phase sensor utilizing non-linear interferometers, wherethe refractive index of the non-linear material is varied.

SUMMARY OF THE INVENTION

Basically and generally expressed, the present invention relies on amethod, where the processing of an optical input signal includes atleast the steps of: 1) excitating with at least part of said inputsignal an optical resonator arrangement that comprises two substantiallyparallel, independent, complex one-pole resonators arranged in a mannerthat that one of said resonators is matched with the input signal andthe other one is non-matched with the input signal, and 2) based onpolarization separating further from the output of said opticalresonator arrangement at least one optical output signal so that bothsaid matched and non-matched resonators of said optical resonatorarrangement contribute to the formation of said linearly polarizedoutput signal.

The method may be carried out in practise, for example, by using asingle optically birefringent resonator, whose output is furtherdirected to a suitably located polarization selective device, thatselects at least one direction of polarization from the output saidbirefringent resonator. In such an embodiment, the matched andnon-matched resonators become formed inside said resonator arrangementdue to the birefringency as two virtually independent resonators.

As will be shown later, the matched and non-matched resonators togetherwith the polarization separating or selective device or means may berealized in several ways and using various types of optical elements.All the optical devices carrying out the method generate different typesof bandpass filtering of the incoming optical signal and provide thususeful tools for various all-optical signal-processing applications.

The significant benefits of the invention include, for example, the factthat the present invention can be used to realize optical devices thatprocess multiple wavelength channels simultaneously, that is to say inparallel. The invention requires that the optical length of onepolarization mode of the resonator arrangement is matched with thewavelength of the incoming light, but it has no temporal signal lengthrequirements, i.e. requirements for the duration regarding the rate ofthe incoming optical data. The data rate is matched by selecting, forexample, suitable amount of birefringence into the resonator. The methodof the invention thus enables parallel processing of multiplewavelengths simultaneously, provided that the resonator length ismatched to all used wavelengths and the birefringence is suitable forthe data rate.

One promising application of the invention can be found in all-opticalclock recovery. However, the parallelism mentioned above holds true forall applications, provided that different wavelengths can later beseparated from each other.

The most profound benefits of the invention vary according to thespecific applications. When used for all-optical clock recovery themethod liberates from the dependency between the resonator length (WDMchannel separation) and the data rate. This freedom opens opportunity tobuild truly parallel systems in practise. When compared to U.S. Pat. No.5,446,573, U.S. Pat. No. 6,028,687, or U.S. Pat. No. 6,338,753, thepresent invention requires smaller number of optical elements and givesthus simplicity of construction and savings in cost. In particular, thepresent invention does not necessarily require the use of any opticallyactive medium (semiconductor-optical amplifier or laser source) toprovide adequate signal levels and operation. In its simplest form, thepresent invention requires only the use of one birefringent resonatorarrangement together with one polarization selective element. Forexample, U.S. Pat. No. 5,446,573 requires a non-linear ring, whichconsists of two laser sources, and four phase-modulators. U.S. Pat. No.6,028,687 requires an optical mux/demux element and at least twosemiconductor optical amplifiers within an optical resonator.

U.S. Pat. No. 6,338,753 requires non-linear interferometer, whoserefractive index is varied with external optical pulses, which inpractice means a sufficient high-energy laser build into a workingsystem. More details of the present invention are set forth in theforegoing description and in the accompanying drawings describingselected embodiments of the invention. The preferred embodiments andpossible variants of the invention will become more apparent to a personskilled in the art also through the appended claims.

It should be understood that in the following the purpose of thetheoretical and mathematical descriptions is merely to function as toolsfor describing the invention better for a person skilled in the art.Therefore, even though the theory might not be completely immaculate inall respects, it nevertheless represents the Applicant's bestunderstanding of the phenomena underlying the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 A basic setup of an optical signal frequency filter according tothe invention.

FIG. 2 Phasor representation of the intracavity electric field.

FIG. 3 Step-pulse response of the intracavity electric field when r=0.99and phase is matched, i.e. φ=0°. The electric field has no imaginarycomponent.

FIG. 4 Step-pulse response of the intracavity electric field when r=0.99and phase mismatch φ=10°. Due to the phase mismatch the field iscomplex. The real and imaginary parts of the electric field oscillatesinusoidally and have phase-shift π/2.

FIG. 5 Transfer functions for φ=0.1 and φ=0 at r=0.999.

FIG. 6 Train of pulses (upper part) with period T=188.5. Frequencydecomposition of the signal (lower part) reveals that it has energy peakat angular frequency 0.1 rad n⁻¹. Dashed line represents theFourier-transformation of rectangular signal, circles show the accurateenergy density position and magnitude of the periodic signal and peaksshow the computational FFT solution of the signal.

FIG. 7 Resonator response for square pulse excitation at angularfrequency φ/3. Real and imaginary parts of the electric field (upperpart) show strong dependence on third harmonic component of the inputexcitation. The intensity of the electric field (lower part), isflattened and does not reflect filtering property of the resonator. Forcomparison one period of the input excitation (dashed line) is shown.

FIG. 8 Resonator has two axis of birefringence, symbolized with axis xand y. The polarization of incoming electric field has angle α withrespect to the axis of birefringence and the polarization beam splitterdivides the out-coming electric field according to angle θ.

FIG. 9 System response with a resonator, where x-axis is matched (φ=0),y-axis is non-phase-matched (φ=0.1), reflectivity is r=0.999, angle ofinput polarization is α=45° and angle of polarization beam splitter PBSis θ=45°. In the upper part the intensity components I_(x), I_(y), andI_(b) are depicted against a single period of excitation E_(in). Onlower part the final output arms show an oscillating signal, whichhappens to be third harmonic of the input pulse frequency.

FIG. 10 Properties of a train of Gaussian pulses with period T=125.7 tobe used as input for a resonator.

FIG. 11 System response to the pulses defined in FIG. 10.

FIG. 12. Properties of a train of Gaussian pulses with period T=100 tobe used as input for a resonator.

FIG. 13 System response to the pulses defined in FIG. 12.

FIG. 14 Burst of random data to be used as input for a resonator.

FIG. 15 System response to the pulses defined in FIG. 14.

FIG. 16 An embodiment of the device according to the invention usingpolarization controllers before and after resonator arrangement.

FIG. 17 A system with parameter values: r=0.99 and θ=75°, is subject tochange of birefringence. This system is excited with a step-function.The system intensity output Bout1 from arm 1 is depicted at threedifferent phase-shift values, namely φ=0, φ₁=10°, and φ₂=20°.

FIG. 18 A miniaturized embodiment of the device according to theinvention.

FIG. 19 An embodiment of the device according to the invention with afirst polarization beam splitter in the front of the setup following twoseparate resonators whose outputs are further combined with anotherpolarization beam splitter.

FIG. 20 An embodiment of the device according to the invention based onthe use of a micro sphere resonator.

FIG. 21 An embodiment of the device according to the invention based onthe use of photonic bandgap structures.

DETAILED DESCRIPTION OF THE INVENTION

In the following the invention is mainly described referring to abirefringent resonator arrangement, where the matched and non-matchedresonators are formed within a single physical cavity due to thebirefringency. However, the invention is not limited to suchembodiments, but the invention may also be realized using separatematched and non-matched resonators. One example of such an embodiment islater given in FIG. 19.

In the FIGS. 1,16,19 and 20 the polarization axes are depicted with“crossed” markings using both solid and dashed lines.

It is known from prior-art, as such, that an optical resonator behavesas an one-pole complex filter. Complex one-pole resonators generally,working either in optical, digital-electronic etc. domain, are able totransform purely real input signal to a complex one. Only when thewavelength of excitation is matched with the resonator, it has lowpassfiltering function with no imaginary parts and therefore, it candirectly be used for signal processing purposes. Alternatively, if thewavelength and rate of data both are matched, then it operates as abandpass filter. However, when the wavelength of excitation does notmatch with the resonator, it always behaves as a complex bandpassfilter. Although real and imaginary parts both pass only a narrow bandof signal spectrum with equal gain, they differ π/2 in phase. While theoptical output is always measured as the square of amplitude (i.e.intensity) and because of the quadratic phase-shift between real andimaginary parts of the signal, the output is flattened out and theresonator cannot be used directly as a bandpass filter (later this willbe shown graphically).

If one could separate real and imaginary parts from each other, orsuppress one of them, the resonator bandpass characteristics couldeffectively be utilized also in case of wavelength mismatch. The currentinvention provides a novel and inventive way of doing this.

The key of the invention lies in the novel way of taking into use thepotential bandpass filtering capabilities of an optical complex one-poleresonator by, 1) excitating with at least part of the input signal anoptical resonator arrangement that comprises two substantially parallel,independent, complex one-pole resonators arranged in a manner that oneof said resonators is matched with the input signal and the other one isnon-matched with the input signal, and 2) based on polarizationseparating further from the output of said optical resonator arrangementat least one optical output signal so that both said matched andnon-matched resonators of said optical resonator arrangement contributeto the formation of said output signal.

As will be shown later, such an arrangement, that may be realized forexample by using a birefringent resonator combined with a polarizationbeam splitter, will be able to amplify the real part of the mismatchedoptical field and suppress the imaginary part. Therefore, the output ofsuch resonator-polarizer setup can be made highly dependent on thefiltering properties of the complex one-pole resonator.

FIG. 1 describes schematically one embodiment of the invention combininga birefringent optical resonator OR with a polarization beam splitterPBS in order to divide the light beam Bin inputted in said system intotwo separate optical arms Bout1, Bout2. This basic setup can be realizedin a variety of different ways, as will be shown later.

In the following some basic principles of optical resonators are firstdiscussed. Real and imaginary part transfer functions of a complexone-pole resonator are presented and an intensity output of theresonator is presented. Especially, the consequence of the birefringenceof the resonator OR and the effect of the inclusion of a polarizationbeam splitter PBS at the resonator output are explained. Finally,selected examples of systems according to the invention, and usingvarious optical excitation forms are presented together with someapplications.

Optical Resonator and Phasors

Amplitude and phase of an electric field inside an optical resonator canbe presented with phasors as depicted in FIG. 2. At time t=0 an electricfield E_(in)(0) is injected into the resonator. After one round-trip theelectric field amplitude is decreased by factor rε[0,1] and hasexperienced a phase-shift φ (in following the phase-shift φ isunderstood to be a modulo of 2π). Additionally, a new field-component isadded giving a total electric field E(1)=re^(iφ)E_(in)(0)+E_(in)(1).After n number of round-trips the intracavity electric field becomes$\begin{matrix}{{{E(n)} = {{\sum\limits_{p = 0}^{n}{\left( {r\quad{\mathbb{e}}^{\mathbb{i}\varphi}} \right)^{p}{E_{in}\left( {n - p} \right)}}} = {{r\quad{\mathbb{e}}^{\mathbb{i}\varphi}{E\left( {n - 1} \right)}} + {E_{in}(n)}}}},} & (1)\end{matrix}$where n=t/t_(n) is the round-trip number and t_(n) is the duration of asingle round-trip. Optical resonator, as a one-pole complex filter, willproduce complex electric field inside the resonator in presence of phasemismatch φ. Buildup of energy under constant injection in an arbitraryresonator (r=0.99) with phase mismatch φ=0° and φ=10° are depicted inFIGS. 3 and 4, respectively.

When phase is matched (φ=0), the energy starts to build up within theresonator and the net electric field has no imaginary part. Now theresonator behaves as a low-pass filter and has time constant τ=l(−cln r)for light intensity, where c is the speed of light within the medium, lis the one-pass length of a Fabry-Perot resonator and r is one-pass lossfactor of light intensity within the resonator.

When phase is not matched, the electric field becomes complex and thephasor draws a converging spiral loop. As visible in FIG. 4, and asknown from prior-art, the real and the imaginary part of a causal signalare connected through Hilbert-transform.

Impulse response of the resonator ish(n)=(re ^(iφ))^(n) u(n)  (2)where u(n) is the step-function (Heaviside function) due to causalnature of the response. Fourier-transform of the impulse response ofreal and imaginary parts give transfer functions $\begin{matrix}{{{T_{Re}(\omega)} = {\frac{1}{2}\left\lbrack {\frac{1}{{\mathbb{i}\omega} - {\ln\left( {r\quad{\mathbb{e}}^{\mathbb{i}\varphi}} \right)}} + \frac{1}{{\mathbb{i}\omega} - {\ln\left( {r\quad{\mathbb{e}}^{- {\mathbb{i}\varphi}}} \right)}}} \right\rbrack}}{and}} & (3) \\{{T_{Im}(\omega)} = {\frac{- i}{2}\left\lbrack {\frac{1}{{\mathbb{i}\omega} - {\ln\left( {r\quad{\mathbb{e}}^{\mathbb{i}\varphi}} \right)}} - \frac{1}{{\mathbb{i}\omega} - {\ln\left( {r\quad{\mathbb{e}}^{- {\mathbb{i}\varphi}}} \right)}}} \right\rbrack}} & (4)\end{matrix}$where ω=2πf=2π/n is an angular frequency expressed in units of rad/n.Transfer functions for φ=0.1 and φ=0 at r=0.999 are illustrated in FIG.5. Bandpass filtering property of the optical resonator at phasemismatch and lowpass property at phase match are evident. By increase ofcavity finesse, i.e. r→1, the filter becomes increasingly selective.

Filtering property of the resonator is now shown in time domain for aperiodic input. Let us assume a train of pulses with period T=188.5 asshown in FIG. 6. Frequency decomposition of the signal reveals that ithas the fundamental angular frequency at 1/30 rad n⁻¹, its secondharmonic is suppressed and it has third harmonic at 0.1 rad n⁻¹. Whensuch signal is fed into a resonator with phase mismatch φ=0.1 andr=0.999 we could expect that real and imaginary parts of the signalreflect this third harmonic component while all other frequencycomponents are highly suppressed. This becomes evident from FIG. 7 upperpart. Because the amplitudes are nearly the same and phase isquadratically shifted between the real and imaginary parts of thesignal, modulus of the complex signal remains approximately constant. Inphasor representation the vector would draw just a circle with nearlyconstant radius. Intensity output I∝EE* is thus flattened as shown inthe lower part of FIG. 7. It is evident that such resonator is hardlyusable as a bandpass filter at wavelength mismatch. The situation wouldchange, however, if either real or imaginary part of the signal could besuppressed.

Optical Resonator Birefringence and Polarization Beam Splitter

When an optical resonator OR is birefringent, the path lengths insidethe resonator are uneven for orthogonal polarization components. Whenthis type of resonator is combined with a polarization beam splitter PBSand circulating electric field has phase-matched and non-phase-matchedcomponents, the imaginary part of the electric field component can besuppressed. In other words, according to the present invention thesystem output can be made strongly dependent on the real part electricfield of the mismatched resonator and such system behaves as an opticalsignal bandpass filter.

Let's define x-axis of the resonator phase-matched and y-axisnon-phase-matched, the amount of mismatch symbolized by φ. Intracavityelectric fields areE _(x)(n)=rE _(x)(n−1)+E _(in,x)(n)  (5)andE _(y)(n)=re ^(iφ) E _(y)(n−1)+E _(in,y)(n).  (6)

We assume that the incoming field has angle α with respect to the axisof birefringence of the resonator OR and the polarization beam splitterPBS divides the outcoming electric field according to angle θ, as shownin FIG. 8. The polarization beam splitter PBS outputs Bout1 and Bout2are labeled below in the equations shortly as optical arms 1 and 2. Thisholds true especially for a polarization beam splitter, but it shouldnoted that the invention is not limited to the use of a polarizarionbeam splitter, but any suitable polarization separating or selectingmeans may be employed. It can be shown that the output intensity of thepolarization beam splitter PBS is $\begin{matrix}{{\begin{pmatrix}I_{1} \\I_{2}\end{pmatrix} = {\begin{pmatrix}{\cos^{2}\theta} & {\sin^{2}\theta} & {\sin\quad 2{\theta/2}} \\{\sin^{2}\theta} & {\cos^{2}\theta} & {{- \sin}\quad 2\quad{\theta/2}}\end{pmatrix}\begin{pmatrix}I_{x} \\I_{y} \\I_{b}\end{pmatrix}}},} & (7)\end{matrix}$where I_(x)=E_(x)E_(x)*, I_(y)=E_(y)E_(y)*, andI_(b)=E_(x)E_(y)*+E_(x)E_(y)=2E_(x)ReE_(y). As present, the oscillating,or beating, term I_(b) is independent on imaginary electric field ImE_(y) when the field E_(x) is matched with the resonator.

In order to show the effect, the same pulse excitation than in previouscase is used with a resonator, where x-axis is matched (φ=0), y-axis isnon-phase-matched (φ=0.1), reflectivity is r=0.999, angle of inputpolarization is α=45° and angle of polarization beam splitter PBS isθ=45°. In the upper part of FIG. 9 the intensity components I_(x),I_(y), and I_(b) are depicted against a single period of excitationE_(in). The oscillating term clearly reflects the absence of imaginarycomponent and the real component of the electric field is amplified byE_(x). On lower part of the FIG. 9, the final output arms show anoscillating signal, which happens to be third harmonic of the inputpulse frequency.

The operation can be proved also for other type of signals. A train ofGaussian pulses with period T=125.7, as depicted in FIG. 10, is fed intothe same resonator than used previously. The second harmonic of thefrequency decomposition is exactly at resonance frequency of theresonator and, therefore, we can expect this second harmonic componentto be passed by the filter. This is exactly what happens as depicted byFIG. 11. If the same train of pulses period slightly changed (T=100) andthus no frequency components at resonance frequency, is fed into thepreviously introduced resonator, the filter output is essentially flat.For this effect, see FIGS. 12 and 13, where said effect becomes obviousfor any person skilled in the art.

Applications, All-Optical Multiwavelength Clock Recovery

One interesting application of the invention can be found in all-opticalclock recovery. When compared to prior art solutions to make all-opticalclock recovery by use of Fabry-Perot resonator, the arrangement of theinvention is superior by its ability to operate simultaneously atmultiple wavelengths. A burst of data (half of bits randomly shut off)with clock period T=62.8 is fed into the resonator OR as shown in FIG.14. The output of the resonator OR is depicted in FIG. 15. For sake ofclarity only arm 1 (Bout1) is plotted. This case illustrates very wellthe properties of the invented system. It selectively filters theangular frequency, which is in match with the resonance frequency of theunmatched polarization component, and in addition to this, it maintainsthe operation for some time depending on resonator finesse. Opticalresonator can be thought as an temporal energy store. Now it stores andmaintains the oscillation it has picked up for a certain time.

In the following an example calculation of making filtering at severalwavelengths simultaneously is given.

We shall assume a set DWDM channels (Δν=100 GHz) operating at λ=1550 nmwavelength region. Data rate at each channel is f=40 GHz. A resonatormedium has refractive index n=1.5. In following we will determine therequired Fabry-Perot resonator length l, degree of birefringence Δn, andone-pass intensity loss factor r for simultaneous operation of thesystem at multiple wavelengths.

Resonator x-modes are set to coincide with channel separation of theDWDM system, i.e., Δν=c/(2nl), which gives the resonator length l=1 mm.The DWDM channels would also be matched with resonator modes when thelength is integer multiple of l, as clear for the persion skilled in theart.

The filtered frequency is dependent on the difference of x- and y-moderefractive indices $\begin{matrix}{{f = \frac{c\quad\Delta\quad n}{\lambda\quad n}},} & (8)\end{matrix}$which determines the required degree of birefringence to be Δn=3.1-10⁻⁴at λ=1550.00 nm.

As present from Eq. 8 the filtered frequency is dependent on thewavelength of light. While different channels have slightly differentwavelength, the filtered frequency is also changed. The difference infiltered signal frequency can be expressed in function of the wavelengthchange $\begin{matrix}{{df} = {{- \frac{c\quad\Delta\quad n}{\lambda^{2}n}}d\quad{\lambda.}}} & (9)\end{matrix}$

The channel separation can be expressed in terms of wavelengthdλ=−λ²dν/c=−0.8 nm, which translates to shift of filtering frequencyaccording to following Table 1.

Passband of a Fabry-Perot resonator is defined as $\begin{matrix}{{\delta\quad v} = {{{\ln\quad r}}{\frac{c}{2\pi\quad n\quad\ell}.}}} & (10)\end{matrix}$

The filter operates for those DWDM channels where the filteringfrequency f shifts stays within the passband of the resonator. Thishappens when df<0.5 δν. TABLE 1 Channel # dλ (nm) df (GHz) 0 0 0 1 −0.80.02 2 −1.6 0.04 10 −8.0 0.21 20 −16 0.41 50 −40 1.03

The passbands and the number of possible simultaneously processedchannels for various loss factors r are presented below in Table 2.TABLE 2 0.5 δν Channel dλ df r (GHz) shift max. # (nm) (GHz) 0.9 1.68 8164.87 1.67 0.99 0.16 7 5.61 0.14 0.998 0.03 1 0.8 0.02

As can be seen, the resonator is capable of processing smaller number ofchannels the lower the loss factor (higher the mirror reflectivity) is.High loss factor (low mirror reflectivity) gives larger number ofchannels to be processed, but with lower capability for recovery (theenergy storage of the resonator is lossy). One has to make a tradeoffbetween the number of processed channels and the capability to recoverthe clock signal.

The situation can be compensated by suitable control of the wavelengthdependence of the birefringence Δn(λ).

One possible setup contains, as depicted schematically in FIG. 16, afirst polarization controller PC1, a Fabry-Perot type fiber opticalresonator OR with ability for a fiber twist, second polarizationcontroller PC2, and a polarization beam splitter PBS.

The first polarization controller PC1 sets the state-of-polarization ofincoming light such that the angle of polarization (azimuth) differsfrom the axis of birefringence of the resonator OR. Should theycompletely coincide, no oscillation may be observed at the output whileall energy is directed either on matched or non-matched resonator mode.

In this embodiment, the fiber optical resonator OR is constructed bycoating cleaved or polished fiber end with high reflectivity dielectriccoating. The fiber length is chosen such that free spectral range (FSR)is matched with the channel spacing of the incoming light. The opticallength of the resonator is regulated in order to keep the x-mode opticallength matched with channels of the incoming light. In the context ofthis invention the aforementioned matching is shortly referred to asmode-locking and in addition to a fiber resonator it applies also toother type of resonators. Instead, or in addition to adjusting thelength of the resonator, mode-locking may also be achieved by tuning thewavelength of the incoming light. The resonator has preferably lowinternal loss and has high reflectivity dielectric mirrors. The degreeof birefringence, and thus the angular frequency of the mismatch, isadjusted by twisting the optical fiber constituting the resonator. Theamount of angular frequency mismatch should be equal with the angularfrequency of the incoming data.

Second polarization controller PC2 sets the state-of-polarization of theresonator OR output such that the polarization beam splitter PBS dividessignals suitably between the two output arms Bout1, Bout2.

If the processed signal has multiple wavelengths (channels), eachchannel should have data frequency matched with the degree of angularmismatch from channel to channel.

The phase may differ. Later one may want to separate channels from eachother, but this speculation and related techniques are beyond the scopeof the present invention.

In the following certain variations and alternatives for the differentparts of the setup carrying out the invention have been listed. Theseapply to the clock recovery application already explained, but are alsoapplicable, mutatis mutandis, to other applications and optical setupsdescribed in this text. Further, one should not consider the inventionto be limited even by these variations and alternatives, but theinvention should be restricted only in the manner indicated by the scopeof the claims appended hereto.

The resonator OR does not need to be of a Fabry-Perot type, but it couldalso be, for example, a fiber loop resonator (preferably with weak inputand output couplings). It may also be a micro-ring, -sphere, -toroid ora photonic bandgap resonator. In such “non-Fabry-Perot” loop or ringtype resonators the optical medium is arranged to form at least partlyclosed optical circle where the light travels substantially only onecommon direction without multiple back-and-forward reflections.

The cavity medium of the resonator OR may constitute of a normal singlemode fiber or polarization maintaining fiber. However, the setup mayalso be build completely without any fiber optic components. Therequired optical functions can be achieved with free-space optics orintegrated optics. Therefore, the resonator OR could be, for example, acoated dielectric rod, a pair of free-space dielectric mirrors or even asemiconductor device. The cavity medium may be simply air or othertransparent gas or liquid instead other optically transparent,dielectric or semiconductor materials. It should be noted, that theinvention is not limited only to visible wavelengths, but it can be usedbroadly with any optical wavelengths that may be shorter or longer thanvisible wavelengths. It is also obvious for a person skilled in the artthat certain coherence of the inputted light is required.

The “mirrors” of the resonator OR may be, for example, dielectric stackmirrors, metallic mirrors, fiber loop mirrors, fiber couplers (notreally a mirror, but input port with weak efficiency) or different typeof waveguide couplers. Further, said mirrors may have differentreflectivities for x- and y-directions. High reflectivity mirrors aretypically desired.

Light is necessarily not coupled into the resonator OR through theaforementioned mirrors, but evanescent coupling, or any other opticalcoupling methods known in the art may also be used. Input coupling maybe realized, for example, with prisms arranged in planar waveguides.

The degree of birefringence in the resonator OR can be adjusted with notonly twist, but also by stress, tension, bend, or other structuraldeformations of the cavity medium.

Temperature and electro-optical effects may also be used to affect thebirefringence of the resonator depending on the cavity medium/material.For example, if the cavity medium is gas or vacuum, then thebirefringency of the resonator may be altered by deformation of the endmirrors or corresponding cavity forming reflectors.

As already mentioned, the resonator arrangement does not need to basedon a birefringent cavity, but instead it may also be formed from twophysically separate resonators one matched with the incoming light andthe other one non-matched with the incoming light. The polarizationstate of the incoming light is required to be such that both of theseresonators participate in the formation of the final output signalavailable after the polarization separating means.

Polarization controllers PC1,PC2 before and after a resonator OR, asshown schematically in FIG. 16, may not be necessary at all if thepolarization of the incoming light is already suitable for the resonatorOR, and the selection angle of the polarization beam splitter PBS iscorrect or it can be adjusted.

Polarization separating means may be a beam splitter PBS or any opticalcomponent or set of components known in the art, which selects orseparates at least one polarization component from the inputted light.It is not even necessary to divide the polarization components of thelight into different optical arms, but one of these components may beabsorbed or otherwise dumped during the separation process. Possibledevices include, but are not limited to the following: Wollaston prism,Glan-Foucault polarizer, Nicol prism, Rochon prism, dielectric coatingpolarizer, wire grid polarizer, polymer based film polarizer, singlepolarization mode transmitting fiber, photonic crystal polarizationseparator. The polarization separation means do not need to be comprisedin a separate optical component, but it/they can be, for example,integrated directly on the output side of the resonator arrangement.

In case when the birefringence of the resonator OR is wavelengthdependent, then the extracted frequency may vary from inputted channelto channel, i.e. data rate is xx GHz for inputted channel one, yy GHzfor channel two etc.

The mode locking may be active or passive. For example, if the lightsource is in vicinity of the resonator OR, the out-leaking light fromresonator OR may be used to feed some light back to the coherent lightsource (typically laser). For the correct operation of the setup,however, the resonator OR is typically required to be substantiallymode-locked with the incoming light/light source.

The excitation of the system according to the invention may be due tochange of the incoming light intensity, change of its polarization,change of the resonator length (so that x-axis also becomesnon-phase-matched), or change of the resonator medium refraction index.The excitation may be generated by the light source, an externalmodulator (modulating light intensity or polarization), or couplingstrength of the input port of the resonator. Also the input portcoupling strength may be modulated. Excitation forms may be periodic:train of square pulses, Gaussian pulses, solitons (sech), ornon-periodic: stream of data . Other type of input signals may also beused. Therefore, the excitation may be due to the properties of theincoming light itself, or the excitation may also be accomplished byaltering the properties of the resonator arrangement.

Applications, Signal Frequency Component Analysis The setup describedschematically in FIG. 16 can be used for input signal RF-componentanalysis when equipped with at least one optical sensor (photodiode) andmeans to read the sensor output. Preferably, photodiodes or other typeof fast optical sensors are arranged to sense the output of at least oneoptical signal Bout1 and/or Bout2. The degree of birefringence of theresonator OR is swept (by e.g. twist) such that the optical frequencyphase mismatch goes step-by-step from 0 to λ. While each opticalfrequency mismatch corresponds to a certain radio frequency of thesignal, the output signal Bout1,Bout2 amplitudes are read during thesweep. One can thus obtain information about signal frequencies of theincoming signal Bin. In this configuration one utilizes the one-polecomplex resonator frequency selectivity for inspection of optical signalquality.

Applications, Higher Harmonic Signal Generation

The setup of the previous cases explained abobe (FIG. 16) can be usedfor optical microwave generation when further equipped with suitableoptical light source. As explained and illustrated in already earlier inthis application, the higher harmonics of, e.g. train of square pulses,can be extracted if the input signal contains higher harmoniccomponents, like some odd harmonic of a train of square pulses.

Applications, Measurement of Birefringence

The filtering property of a resonator system according to the inventionis dependent on the degree of birefringence in the optical resonator OR. This property can be used for the measurement of minute birefringencein the following way.

When the system is excited with an optical signal Bin, which includesmultiple ranges of frequencies, or continuum of frequencies, the changeof filtering frequency can be determined from frequency outputoscillations. An example of a signal with continuum of frequencies is astep-function, and a signal with multiple frequencies is, for example, aperiodic sinc signal.

A system with parameter values: r=0.99 and θ=75°, is subject to thechange of birefringence. This system is excited with an inputtedstep-function. The system intensity output Bout1 from arm 1 is depictedin FIG. 17 at three different phase-shift values, namely φ₀=0, φ₁=10°,and φ₂=20°. As present, the oscillating frequencies are dependent on thechanges of the birefringence. The oscillation will dampen out while theexcitation continues unchanged.

If we know the initial state of the studied optical system, i.e., itsbirefringence, the change of oscillation frequency is an indication ofthe change of birefringence. While the oscillation will dampen out oncourse of time, the excitation should be repeated at some time interval.

An alternative way to measure birefringence is to excite the system witha periodic signal, whose fundamental frequency, or some of itsharmonics, coinside with the filtering frequency of the resonatorsystem. The excitation frequency is actively tracking the filtertransmission maximum by, e.g., using a voltage controlled oscillator. Ifthe filter output amplitude decreases, the excitation frequency isadjusted such that the output amplitude reaches again its maximum. Thus,the frequency of excitation is an indication of the birefringence of theoptical system.

Applications, Miniaturized Embodiment

The invention may also be realized as a miniaturized optical arrangementdescribed schematically in FIG. 18. Optical resonator OR may be realizedas a miniaturized (just a few mm in dimensions) Fabry-Perot etalon whoseboth optical ends have been coated with a dielectric reflectivecoatings. Polarization selective element PBS is a miniature sizepolarization beam splitter or polymer film polarizer. According to oneembodiment, the PBS could be cemented or integrated directly right afterthe optical resonator OR. In FIG. 18 only one optical arm of the PBS isutilized, and the useful light is amplified with semiconductor opticalamplifier SOA and then directed to output. The input and output couldnaturally comprise additional optical elements, like lenses for focusingor waveguides for light transmission. The optical resonator ORtemperature is regulated with one or more thermoelectric coolers TEC1,which stabilize the optical resonator OR temperature to a desired value.This is needed to keep the resonator transmission modes stable and theoptical resonator OR mode-locked with the wavelengths of the incominglight. The device package P may include still further thermoelectriccoolers or temperature regulators, although they have not been depictedin FIG. 18. The birefringence of the optical resonator OR can in fact becontrolled with, e.g., two or more thermoelectric coolers. When two ormore temperature regulators are connected to the optical resonator OR,they can be used to create a temperature gradient, which inducesbirefringence on the optical resonator OR. By controlling thetemperature difference and the average temperature, one maysimultaneously adjust the optical resonator OR birefringence and themode-locking property. Alternatively, a pair of electrodes, whoseelectric field induce and control the amount of birefringence, can alsobe used for birefringence control.

Other Applications and Further Variations

The invention can be used for wide variety of applications related toall-optical filtering. Because of the auto-regression nature , i.e.“memory” of the filter, filtering does not however apply for very fastchanges. For certain applications, this may be a desired feature insteadof a limitation.

Again, it needs to be emphasized, that while the invention has beenshown and described here with respect to a few selected embodiments, itshould be understood that these embodiments are only examples, and thata person skilled in the art could construct other embodiments utilizingtechnical details other than those specifically disclosed herein whilestill remaining within the spirit and scope of the present invention. Itshould therefore be understood that various omissions and substitutionsand changes in the optical design of the resonator arrangement,polarization selective element and related optical components couplinglight in and out from said components, as well as in the mutualorganisation and operation of the same, may be made by those skilled inthe art without departing from the spirit of the invention.

For example, according to the invention it is also possible to arrange afirst polarization beam splitter PBS1 in the beginning of the setup,then two resonators OR1 and OR2 (one matched and other non-matched),whose outputs are further combined with another polarization beamsplitter PBS2. This embodiment is schematically shown in FIG. 19. Thepolarization axes of beam splitters PBS1 and PBS2 are depicted withsolid line. The polarization axes of PBS1, which determines theprinciple polarization axis of the whole system, are illustrated withdashed lines in other parts of the system. Components M1-M3 are mirrors.Here, instead of a single birefringent resonator arrangement theresonator arrangement comprises two physically separate individualresonators OR1 and OR2.

The setup according to the invention has been shown in the aboveexamples to operate in transmission. However, it should be obvious for aperson skilled in the art that embodiments also operating in reflectionare possible. The resonator arrangement OR may thus be realized in sucha way that the light inputted in the resonator through a port (typicallya mirror) outputs from the same port. In the above given examples theresonator has been arranged with separate ports for input and output.

As a general way of optimizing the performance of the optical setupaccording to the invention, the angle of the linear polarization of theincoming light may be adjusted respect to the polarization axes of theresonator arrangement so that the oscillations at the output aremaximized. Similarly, the angle of the polarization separating elementafter the resonator arrangement may be adjusted for the same purpose.

A light amplifying optical element or light amplifying means may bearranged in front of the resonator arrangement, after it or within it.Preferably, such an element is arranged between the resonator and thepolarization separating means. Suitable light amplifying elementsinclude, for example, active optical fibers and other active waveguides,which are typically based on the use of rare-earth element dopedmaterials. Another possibility is the use of various semiconductoroptical amplifier (SOA) devices as shown in FIG. 18.

FIG. 20 describes schematically still one possible embodiment of thedevice according to the invention based on the use of a micro sphereresonator. A waveguide is arranged to couple the incoming light Bin intothe resonator OR, and another waveguide couples the light out from theresonator to a polarizing beam splitter PBS.

FIG. 21 depicts how the device according to the invention may bemanufactured using photonic bandgap structures. Expressed in moregeneral terms, the device may be manufactured by use of light scatteringperiodic microstructures. Such a device can be built in the size ofminimum of few tens of micrometers. A solid-state substrate medium (likeglass, silicon, or suitable polymer) can serve as the base for periodicmicrostructures. The incoming light Bin can be guided along the missingrow of microstructures. Optical resonator OR can be formed using two ormore microstructures, or it may be formed in the shape of a ring, andthe outcoupled light can be guided to a photonic crystal polarizationbeam splitter PBS. FIG. 21 is purely illustrative and microstructureform, size, pitch, aspect ratio, etc. are dependent on used wavelengthand optical design.

1. A method in optical signal processing of an optical input signal,wherein the processing of said input signal comprises: excitating withat least part of said input signal an optical resonator arrangement thatcomprises at least two substantially parallel, independent, complexone-pole resonators arranged in a manner that the first one of saidresonators is matched with the wavelength of the input signal and thesecond one is non-matched with the wavelength of the input signal, andforming at least one optical output signal so that both said first andsecond resonators of said optical resonator arrangement contribute tothe formation of said output signal.
 2. The method according to claim 1,wherein the imaginary part of the non-matched electric fieldcorresponding to said second resonator is supressed.
 3. The methodaccording to claim 1, wherein the optical input signal is substantiallycoherent and mode-locked with the optical resonator arrangement.
 4. Themethod according to claim 1, wherein said resonator arrangement issubstantially a a single resonator cavity where said independent firstand second resonators are formed as intracavity resonators within saidsingle cavity based on the birefringency of said single cavity.
 5. Themethod according to claim 1, wherein said method is applied to generatea higher frequency optical output signal from a lower frequency opticalinput signal.
 6. The method according to claim 1, wherein said method isapplied to all-optical clock recovery.
 7. The method according to claim1, wherein said method is applied to analyze the signal frequencycomponents of an optical input signal.
 8. The method according to claim5, wherein said method is applied to generate output signal in microwaveor higher frequency range.
 9. The method according to claim 4, whereinsaid method is applied to the measurement of birefringency of saidoptical resonator arrangement.
 10. An optical device for optical signalprocessing of an optical input signal, wherein for the processing ofsaid input signal the device comprises: an optical resonator arrangementexcitated with at least part of said input signal and comprising atleast two, substantially parallel, independent, complex one-poleresonators arranged in a manner that the first one of said resonators ismatched with the wavelength of the input signal and the second one isnon-matched with the wavelength of the input signal, and combining meansto combine the outputs.
 11. The optical device according to claim 10,wherein the imaginary part of the non- matched electric fieldcorresponding to said second resonator is arranged to be supressed. 12.The optical device according to claim 10, wherein the optical inputsignal is arranged to be substantially coherent and mode-locked with theoptical resonator arrangement.
 13. The optical device according to claim10, wherein said resonator arrangement is substantially a singleresonator cavity where said independent first and second resonators areformed as intracavity resonators within said single cavity based on thebirefringency of the said single cavity.
 14. The optical deviceaccording to claim 10, wherein said optical resonator arrangementcomprises one or more Fabry-Perot type resonators in which the opticalmedium of such a single resonator is arranged between cavity formingreflecting means.
 15. The optical device according to claim 10, whereinsaid optical resonator arrangement comprises one or more loop or ringtype resonators in which the optical medium of such a single resonatoris arranged to form at least partly closed optical circle.
 16. Theoptical device according to claim 14, wherein said optical medium partlyor completely consists of one of the following materials or of theircombination: solid material, liquid material, gaseous material orvacuum.
 17. The optical device according to claim 16, wherein said solidmaterial is dielectric material or semiconductor material.
 18. Theoptical device according to claim 14, wherein said optical resonatorarrangement comprises at least one fiber or waveguide resonator.
 19. Theoptical device according to claim 10, wherein said polarizationseparating means is/are based on the use of one or more of the followingoptical items: Wollaston prism, Glan-Foucault polarizer, Nicol prism,Rochon prism, dielectric coating polarizer, wire grid polarizer, polymerbased film polarizer, single polarization mode transmitting fiber orphotonic crystal polarization separator.
 20. The optical deviceaccording to claim 19, wherein said polarization separating means is apolarization beam splitter or a polarization beam selector.
 21. Theoptical device according to claim 10, wherein said device furthercomprises means for altering the polarization state of the light in oneor more of the following positions: before entering the resonatorarrangement, within the resonator arrangement, after the resonatorarrangement but before the polarization separating means, within thepolarization separating means or after the polarization separatingmeans.
 22. The optical device according to claim 10, wherein said devicefurther comprises active light amplifying means.
 23. The optical deviceaccording to claim 22, wherein said active light amplifying meanscomprise a semiconductor optical amplifier.
 24. The optical deviceaccording to claim 22, wherein said active light amplifying meanscomprise a rare-earth doped waveguide.
 25. The optical device accordingto claim 10, wherein said device is partly or completely manufacturedfrom miniaturized optical components.
 26. The optical device accordingto claim 10, wherein said device is partly or completely manufactured byuse of light scattering periodic microstructures.